Effect of Cosmetic Ceramics on Fracture Toughness of All-Ceramic Restorations

Objectives: The use of zirconia as a framework for prosthetic restorations is increasing due to its favorable mechanical properties. Zirconia also has remarkable aesthetic properties when used as a framework and covered with a layer of cosmetic ceramic. The aim of this study was to compare the fracture toughness of three types of aesthetic ceramics, namely VITA VM®9, ceraMotion® Zr, and IPS e.max® Ceram. Materials and Methods: Three groups of aesthetic ceramics (n=10) were subjected to three-point bending tests. The force leading to fracture was recorded for each sample to measure the impact of the ceramic type on the solidity of the framework. The type of fracture has not been studied in this work. One-way analysis of variance (ANOVA) was used to statistically analyze the results. Results: The statistical analysis showed significantly different fracture toughness values among the three groups. IPS e.max® showed the lowest fracture toughness (25.42 MPa) compared to VITA VM®9 and ceraMotion® Zr (respectively 40.39 MPa; P<0.001, and 48.78 MPa; P<0.005). Conclusions: Within the limitations of the present study, it can be concluded that aesthetic ceramics play an important role in the fracture toughness of all-ceramic restorations.


INTRODUCTION
Due to the development of new fabrication techniques and technologies such as computeraided design/computer-aided manufacturing (CAD/CAM), the use of zirconia-based ceramic, as a restorative dental material, is strongly growing because of its superior mechanical properties [1]. Since restorative techniques are constantly improving, dental restorations should match the increasing demands for aesthetic and durable restorations [1]. Over the years, many inconveniences have been raised by ceramometallic restorations, most of them due to the opaque metallic under layer with average aesthetic results such as gradual gingival discoloration in the anterior buccal zone. In addition, ceramo-metallic restorations are prone to corrosion [1]. Since 1958, manufacturers have been developing ceramo-ceramic restorations as an alternative to metal-ceramics. Currently, biomaterial studies in the field of ceramo-ceramics have achieved significant results [2,3]. Although many different materials are now available for the ceramic infrastructure, clinical experience has shown that only two of these materials match the main criteria of mechanical resistance, aesthetics, and ease of processing, namely lithium disilicate and zirconia [2,4,5]. On the other hand, to ensure dental prosthesis sustainability, the veneering ceramic should reach high levels of aesthetic potential and a great reliability [3]. As zirconia-based structures www are usually combined with veneering ceramics in all-ceramic restorations, the purpose of this study was to explore the mechanical behavior of three commercial ceramic veneers in the shape of bilayered zirconia-veneer specimens under threepoint bending tests. Comparative studies of the three selected ceramics were also carried out. . The sample holder included two support rollers, separated by a 20-mm distance, and one loading roller. All the specimens were tested at a crosshead speed of 1.5±0.5 mm/minute until the breaking point was reached. The loads at fracture point were analyzed by using one-way analysis of variance (ANOVA) with a significance level of 5%. Calculations and statistical analyses were performed by using SPSS version 23 software program (IBM Co., Chicago, IL, USA).

RESULTS
A comparative analysis of the fracture toughness of the three veneering ceramics adhered to zirconia is presented in this section. The fracture toughness values of each group are presented in Figure 1 and

DISCUSSION
Many parameters can influence the zirconia coreveneer ceramic interface such as the core roughness and surface energy, the presence of defects (bubbles) at the interface, wettability and viscosity of the veneering ceramic, the stress induced during the cooling phase due to the TEC (thermal expansion coefficient) mismatch between the veneer and core, and the flexural strength of the ceramic [3].
Three veneering ceramics adhered to zirconia were tested in the present study: a low-fusing glass ceramic (ceraMotion® Zr), a feldspathic high-fusing ceramic (VITA VM®9), and a lowfusing nanofluorapatite glass-ceramic (IPS e.max®) with high-fusing liners. The properties of these ceramics are summarized in Table 2.
To eliminate the influence of surface roughness and surface energy of the framework, the same zirconia was used with each of the veneering ceramics: the zirconia bars were directly layered without sandblasting or grinding after sintering. No long-term cooling was performed in this study. The mechanism of ceramic-ceramic adhesion is not as clear as that of the metal-ceramic adhesion. When veneering ceramics are applied to alloys, the mechanical retention and chemical bond between the ceramic and oxide layer play a predominant role in adhesion [7,8]. The bonding mechanism between the veneer ceramic and zirconia www.  [9] showed that mechanical surface treatments such as sandblasting do not improve the adhesion between the veneering ceramic and zirconia. Within the cooling phase, the TEC mismatch between the veneering ceramic and zirconia core leads to the development of residual stresses. The concentration of this stress at the interface can lead to debonding [10]. Due to the viscoelastic properties of the veneering ceramic, a gradient of stress appears in the ceramic at glass transition temperature (Tg), and the ceramic becomes solid [10]. A TEC slightly lower than that of the framework is recommended for the veneering ceramic, which leads to a positive mismatch inducing a compressive stress in the veneering ceramic [10,11]. Stress distribution in a bilayered structure is not uniform; the compressive stress distributed in the ceramic layer is higher at the core-veneer interface and decreases toward the veneering surface leading to a slight tensile stress [10]. A tensile stress is not recommended because it reduces the strength of the veneering ceramic and can induce cracks [10].
The compressive stress generated in the veneering ceramic can strengthen the bilayered structure [4,5]. The degree of the compressive stress in the veneering layer influences the level of flexural strength [11]. Fischer et al [11] demonstrated that the TEC and Tg can influence the adhesion between the bilayered ceramic and zirconia. In this study, the TEC variations of the veneering ceramics between 25°C and 500°C were not significantly different for VITA VM®9 and ceraMotion® Zr ceramics; the values ranged from 9.0×10-6°C-1 to 9.2×10-6°C-1. For IPS e.max®, the TEC was slightly higher (9.5×10-6°C-1). The Tg ranged from 490°C (IPS e.max®) to 600°C (VITA VM®9) ( Table 2). A complex residual stress can be generated in bilayered restorations by the firing processes of the veneering porcelain. Since a slow cooling can negatively affect the flexural strength of bilayered zirconia core and veneering ceramic [12], rapid cooling was performed in this study for all the veneering ceramics. Stress is generated by TEC mismatch, Tg level, and viscoelastic behavior of the ceramic influencing the level of stress [13]. The IPS e.max® with the highest TEC and the lowest Tg presents the lowest adhesion values when applied to noncolored zirconia. Despite the difference in the TEC and Tg, other parameters such as viscoelasticity, wettability, and volume shrinkage of the veneering ceramic also have to be taken into consideration [13]. CeraMotion® Zr, which exhibits the highest bonding strength when applied to zirconia, is a glass with a low firing temperature, whereas VITA VM®9, which is a feldspathic ceramic with a  [19] showed different values from ours when they compared ceraMotion® Zr and IPS e.max® ceramics. This difference may be attributed to the protocol they used which included the application of a liner on IPS e.max® ceramic [19].
A search of the literature shows that the mechanism of bonding between the zirconia and veneering ceramics has remained unknown, and the bond strength between the zirconia and porcelain is still lower than that between metal and porcelain [20]. Our results show that the type of the veneering ceramic plays a significant role in the failure mechanisms of all-ceramic restorations. The adhesion between ceramic and zirconia framework is still an issue influencing the long-term success of prosthetic restorations. The best combinations of core and veneering ceramics should be further studied. Also, clinical studies with larger sample sizes and longer followup periods are required to investigate the possible influencing factors that may lead to technical failures.

CONCLUSION
Our results indicate that more attention should be paid to the choice of veneering ceramic for dental crowns and bridges since significant differences have been found in the fracture toughness of different veneering ceramics.